Esashi Earth Tides Station

Esashi Earth Tides Station was founded as an observation station of the Earth tides. The observations have been conducted since June 1979. The Earth tides are caused by the gravity forces of the Moon and the Sun. They emerge as the surface displacement, most evidently in the vertical direction of about several tens of centimeters, but also as small perturbation in strain and tilt. The measurements of the tides are carried out by using quartz-tube strainmeters and water-tube tiltmeters in a mine whose horizontal length is 250 meters. The strainmeters can measure the change of the strain of the ground with an accuracy of 1 nm/m (strain of 10-9). Observational results reveal how stiff the Earth is and fluid-core resonance which is related to the nutation. The observed data also contribute to the study of the crustal movement. An observation point of the International Gravity Standardization Net is also set up inside the facility.